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JEE Main levelSingle correct optionDifficulty 2 of 3g1213

Which of the following carbocations is the least stable?

Abicyclo[2.2.1]hept-1-yl cation (cation at a bridgehead carbon of norbornane)
Bcyclohexyl cation
Ctert-butyl cation
D2-propyl (isopropyl) cation

Answer: A

A carbocation needs a planar, sp2 centre with an empty p orbital. A bridgehead carbon of the rigid bicyclo[2.2.1] skeleton cannot become planar, and the cation is also not stabilised by hyperconjugation efficiently. It is therefore far less stable than the others, which can all flatten.

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